1. Field of the Invention
The present invention relates to a ventilator for the ventilation of a patient as well as a control unit therefrom and to a method for operating a ventilator.
2. Description of the Prior Art
The functional residual capacity (FRC) of a person is the volume of the lung after expiration at normal pressure. End-expiratory lung volume (EELV) is defined as the volume of gas that remains in the respiratory passages of a patient that is being mechanically ventilated, at the end of a normal expiration. In the case of an elevated positive end expiratory pressure (PEEP), that is, if the end expiratory pressure is higher than the normal pressure, the end expiratory lung volume will be greater than the functional residual capacity. For ZEEP (zero end expiratory pressure, that is, PEEP=0) FRC is equal to EELV.
From a clinical point of view the end-expiratory lung volume is useful, since an abnormally low value may indicate that a part of the lung has collapsed and does not participate in the gas exchange in the lung. Therefore, a simple way of determining the end-expiratory lung volume would be desired, to enable the study of a patient's development over time, for example by determining the end-expiratory lung volume measurement once an hour. Such measurements might also be carried out before and after attempts to open parts of the lung that have been collapsed, a so called lung recruitment maneuver. Thus, measurements of the end-expiratory lung volume could be a complement to other measurement values, such as arterial O2 saturation, which provides valuable information about the ventilation of the patient.
In research applications the washout of an inert gas is used to determine the end-expiratory lung volume. It has been more difficult to find a method that is acceptable for clinical use.
Olegård et al.: “Estimation of Functional Residual Capacity at the Bedside Using Standard Monitoring Equipment: A Modified Nitrogen Washout/Washin Technique Requiring a Small Change of the Inspired Oxygen Fraction”, Anesth Analg 2005; 101:206-12 discloses a method for estimating the FRC or EELV by means of nitrogen washout. Such a method can provide satisfactory results using changes of O2 level in the breathing gas of approximately 10% and procedure duration of approximately 4 minutes, which has been found to be acceptable even in critically ill patients. In this article, the calculations are based on the difference in inspiratory N2 concentration between the start and end of a washout procedure.
The use of O2 or N2 as an indicator gas is advantageous, since these gases are already available from the ventilator, and also are not harmful to the patient.